As a demand for faster operating speed for personal computers and workstations has been increasing in recent years, there has been an increasing effort to achieve faster operating speed of a large scale integrated (LSI) circuit for computation such as a central processing unit (CPU) or a digital signal processor (DSP). Faster operating speed, i.e., a clock frequency, of such an LSI leads to an increase in a heating amount produced by the LSI. Such heating of the LSI may result in thermal runaway and affect surrounding circuits.
An example of a technique for cooling an LSI is an air cooling method using a cooling fan. In this method, for example, a cooling fan is disposed to face a surface of the LSI and blows cooling air to the surface of the LSI.
In many cases, a 3-phase brushless DC motor is used as a cooling fan. The 3-phase brushless DC motor (hereinafter, referred to simply as a “fan motor”) is controlled by detecting a position of a rotor of the fan motor and sequentially changing conduction phases based on the position of the rotor.
As methods of driving a fan motor, a method of driving a fan motor using a hall sensor and a method of driving a fan motor using back electromotive force generated by a coil of the fan motor have been known. The driving method using a hall sensor is advantageous in that a position of a rotor can be accurately detected but disadvantageous in that the cost is increased due to the hall sensor. Further, a fan motor cannot be properly controlled when there is an error in operating the hall sensor.
The driving method using back electromotive force does not require a hall sensor, resulting in a low cost. Further, the driving method using the back electromotive force resolves shortcomings that a fan motor cannot be controlled in case of an error of a hall sensor. In this method, however, in order to detect the back electromotive force, voltage applied to a coil needs to be stopped during a non-conduction period including a timing at which a zero-crossing occurs to maintain a high impedance state. A driving waveform of a fan motor may be distorted due to the non-conduction period. This may lead to a noise.
A device for driving a fan motor may need to drive the fan motor with an appropriate sequence based on a state of the fan motor when the fan motor starts to be driven after power is supplied. That is, when the fan motor starts to drive, the fan motor may be in a stopped state, a forward idle rotation state in which the fan motor is idly rotating in a forward direction due to rotational inertia of a previous driving state of the fan motor, or a reverse idle rotation state in which the fan motor is idly rotating in a reverse direction due to a wind from the outside.
Thus, the device for driving a fan motor is required to have a function of detecting a state of a fan motor when the fan motor starts to be driven. This function may also be required in a multi-phase brushless DC motor, as well as in a fan motor.